The set of 'Castle' must be a really fun place to work -- at least, that's the impression we got after chatting with Laura Prepon about her guest spot on tonight's episode.

"I love the show, and I love the people who run it and work on it," she gushed when TV Squad caught up with her a couple of days before Christmas.

For one thing, they have peanut butter pretzels at craft service. For another, although Nathan Fillion mocked her for bungling the rehearsal of a key kissing scene, she couldn't say enough nice things about the guy.

Prepon's character throws a wrench in the Castle-Beckett dynamic when she shows up to shadow the duo -- she's the Hollywood actress cast as police detective Nikki Heat, the character Castle based on Beckett. Got that? Prepon's playing an actress in the movie within the show that's based on the book within the show that's based on a character within the show. And let's just say that Beckett does not appreciate her presence.

Read on for more info about that darn kissing scene, playing the vapid Hollywood type, and how those peanut butter pretzels fit into the equation.

What's your craziest story from filming?
I'm looking at these peanut butter pretzels right now -- you know those little pretzel nuggets with the peanut butter in them? So Nathan and I had to do this scene where we kiss. I never eat craft service, but right before that scene, I didn't know that we were actually going to do the kiss in the rehearsal and I discovered peanut butter pretzels at craft service. I was eating them and it was like, "Oh, we're doing the kissing scene now!"

I was like "Nathan, I'm really sorry, I just discovered that you have peanut butter pretzels." The next day he got me this huge, Costco-sized box of all these peanut butter pretzels and he was like, "This is for the next time you have a kissing scene so your co-star has to know what I went through." It was so hilarious. That's the type of people they are -- they just joke around the whole time.

So there are definitely sparks flying between you and Castle?
Yeah. There's a little bit of a love triangle thing going on. If anything, it makes him kind of realize that he's into Beckett or whatever. I don't want to give too much away but it's really cool.

You're playing an actress starring in the movie based on the book within the show. That's a lot of layers.
My character is very Method, so I basically follow Beckett around, which in turn means Nathan around too, and basically start mimicking the way she acts. Halfway through the show, I show up in a brown wig and I'm dressed like her ... I've been thinking about going brunette and I got to wear a brunette wig in it, so that was really cool.

Nathan has an interesting storyline because he's obviously in love with Beckett and it's this unspoken thing, and then Nikki Heat shows up looking like Beckett and it's pretty funny. There's a lot of cool stuff going on.

How does Beckett handle the 'Single White Female' situation that's happening to her?
Some scenes she will notice that I'm observing her or acting like her. I think if someone was doing that to me I'd be like, "What are you doing?" But it all works out. I don't want to give away the story, but in the end my character really redeems herself and Beckett respects her.

It's fun to play a cliche of a Hollywood actress because I don't really hang out with that type of person in real life and do all the cliche stuff that you hear about. In the first scene you see this really bad overacting, and then you see my character show up to the set. The show opens with my character doing a screen test for the movie. She's all glamorous and everything, holding a gun and telling this guy to drop it and all that stuff.

Had you seen the show before you read the script?
Yeah, and I really like it. It's not just a procedural -- there's so many fun things that go on in the bullpen, and there's the romantic story between Castle and Beckett. It's not just a straight-up procedural, solve-the-case show. That almost falls in the background behind what is going on with the characters.

It's really funny too.
Yeah, it's really funny. Nathan is hilarious, and the whole side banter that goes on with Esposito and Ryan is hilarious. It's just so much fun. We had a blast.

Did you read 'Heat Wave'?
No, but I have both books at my house! They actually sent them to my house yesterday. I want to read them.